Optical communications system

ABSTRACT

The present invention relates to an optical communications system that allows improving OSNR while suppressing the power increase of pumping light for distributed Raman amplification. In the optical communications system, an optical fiber is laid in a transmission section between a transmitter station (or repeater station) and a receiver station (or repeater station), and optical signals are transmitted from the transmitter station to the receiver station via the optical fiber. In the optical communications system, pumping light for Raman amplification, outputted by a pumping light source provided in the receiver station, is fed into the optical fiber via an optical coupler, and the optical signals are distributed-Raman-amplified in the optical fiber. The transmission loss and the effective area of the optical fiber satisfy, at the wavelength of 1550 nm, a predetermined relationship.

This is a continuation application of prior application Ser. No.12/850,669, filed on Aug. 5, 2010 now U.S. Pat. No. 8,145,024, which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical communications system inwhich optical signals are transmitted via an optical fiber laid in atransmission section.

2. Related Background of the Invention

In order to reduce the number of repeater stations by elongating eachtransmission section and to transmit large volumes of information,optical SN ratio (OSNR) is preferably large, in optical communicationssystems in which optical signals are transmitted via an optical fiberlaid in a transmission section. In order to increase OSNR, transmissionloss in the optical fiber are preferably small, and the effective areaof the optical fiber is preferably large.

Jianjun Yu, et al., ECOC 2008, Th3.E2 (Document 1) disclose results of atransmission experiment that achieved a total transmission capacity of17 Tbps with 114 Gbps×161 wavelengths by 8PSK modulation, using anoptical fiber SMF-28ULL (by Corning) 662 km long and having an averageloss of 0.169 dB/km at a wavelength of 1550 nm.

The Corning optical fiber SMF-28ULL catalogue (ver. August 2008)(Document 2) indicates that the central value of the mode field diameterof the Corning optical fiber SMF-28ULL is 10.7 μm at the wavelength of1550 nm. The effective area of the optical fiber SMF-28ULL is 90 μm²,assuming a Gaussian field distribution.

G. Charlet, et al., ECOC 2008, Th3.E3 (Document 3) disclose results of atransmission experiment in which there was realized a total transmissioncapacity of 3.2 Tbps with 40 Gbps×81 wavelengths by BPSK modulation,over a transmission distance of 11520 km, with 18 recirculations in afiber loop 640 km long, using an optical fiber having a loss of 0.184dB/km and an effective area of 120 μm².

K. Nagayama, et al., Electronics Letters, 26 Sep. 2002, Vol. 38, No. 20(Document 4) disclose an optical fiber having, at the wavelength of 1550nm, a transmission loss of 0.15 dB/km and an effective area of 118 μm².

U.S. Patent Application aid-Open No. 2008/0279515 (Document 5) disclosesan optical fiber having, at the wavelength of 1550 nm, a transmissionloss of 0.16 dB/km and an effective area of 208 μm².

In optical communications systems, optical signals are amplified by wayof optical amplifiers, in order to compensate the optical signal lossincurred during the optical signals propagate through the optical fiberthat is laid in transmission sections. From the viewpoint of the noisefigure (NF) of the optical amplifier, the optical signals in an opticalfiber laid in a transmission section are preferablydistributed-Raman-amplified.

SUMMARY OF THE INVENTION

The present inventors have examined conventional optical communicationssystems, and as a result, have discovered the following problems. Thatis, in conventional optical communications systems, specifically,increasing the effective area of an optical fiber used in a transmissionmedium allows ordinarily increasing the input power into the opticalfiber. While doing so contributes to improving OSNR, it also entails thepower increasing of the pumping light for distributed Ramanamplification, which is problematic.

The present invention has been developed to eliminate the problemsdescribed above. It is an object of the present invention to provide anoptical communications system that allows improving OSNR whilesuppressing the power increase of pumping light for distributed-Ramanamplification.

The optical communications system according to the present inventioncomprises a transmitter station, one or more repeater stations, areceiver station, and an optical fiber laid in transmission sectionspositioned between the transmitter station (or repeater station) and thereceiver station (or repeater station). In particular, the optical fiberlaid in any transmission section at least between the transmitterstation and the receiver station, between the transmitter station and arepeater station, between repeater stations, or between a repeaterstation and the receiver station, transmits optical signals, and theoptical signals are distributed-Raman-amplified. In particular, atransmission loss α (dB/km) and an effective area A_(eff) (μm²) of theoptical fiber satisfy, at a wavelength of 1550 nm, the followingrelationship equations (1a) to (1c):α≈−0.001·A _(eff)+0.27  (1a)0.13≦α≦0.15  (1b)A_(eff)≧120  (1c)

In the optical communications system according to the present invention,the optical fiber preferably include, as a first configuration, acentral core extending along a predetermined axis, and a claddingsurrounding the central core and having a refractive index lower thanthat of the central core. In the first configuration, the central coreof the optical fiber is preferably comprised of pure silica glass, orsilica glass obtained by doping pure silica glass with at least one ofP₂O₅ of 1 mol % or more but 10 mol % or less, Cl of less than 2000 molppm, F of 2000 mol ppm or more but 10000 mol ppm or less, and A₂O (whereA is an alkali metal element) of 1 mol ppm or more but 10000 mol ppm orless. The pure silica glass contains Cl of 2000 mol ppm or more but20000 mol ppm or less that is incorporated in the dehydration processduring a fiber fabrication. Suitable alkali metal elements in the A₂Oare Na, K, Rb and Cs.

In the optical communications system according to the present invention,the optical fiber may include, as a second configuration, a central corehaving the above-described structure, a first cladding surrounding theouter periphery of the central core, a second cladding surrounding anouter periphery of the first cladding, and a third cladding surroundingan outer periphery of the second cladding. Regarding refractive indicesof the central core and the first to third cladding, it is preferablethat the refractive index of the central core is highest and therefractive index of the second cladding is lowest. The second claddingof the optical fiber is preferably comprised of silica glass doped withF element whose amount is more than a doping amount of F element in eachof the first cladding and the third cladding (when there is no doping ofF element, the doping amount of F element includes 0), or silica glassin which a plurality of voids extending in a fiber axial direction(optical axis direction).

The optical communications system of the present invention haspreferably a structure in which optical signals are transmitted inaccordance with a multilevel modulation scheme of four or more levels(for instance, QPSK or 16-QAM).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the configuration of an embodiment of anoptical communications system according to the present invention;

FIG. 2 is a graph showing a relationship at a wavelength of 1550 nmbetween transmission loss, effective area and OSNR of optical fibers;

FIG. 3 is a graph showing a relationship at the wavelength of 1550 nmbetween transmission loss, effective area and power of pumping light forRaman amplification of optical fibers;

FIG. 4A shows a cross-sectional structure of an optical fiber accordingto a first example that can be applied to the optical communicationssystem according to the present embodiment, and FIG. 4B shoes arefractive index profile thereof;

FIG. 5A shows a cross-sectional structure of an optical fiber (secondconfiguration) according to a second example that can be applied to anoptical communications system according to the present embodiment, andFIG. 5B shows a refractive index profile thereof; and

FIG. 6 is a cross-sectional view showing a third configuration of anoptical fiber according to the second example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of an optical communications systemaccording to the present invention will be explained in detail withreference to FIGS. 1 to 3, 4A to 5B and 6. In the description of thedrawings, identical or corresponding components are designated by thesame reference numerals, and overlapping description is omitted.

FIG. 1 is a view showing the configuration of an optical communicationssystem 1 according to the present embodiment. The optical communicationssystem 1 according to the present embodiment comprises a transmitterstation, one or more repeater stations, a receiver station, and anoptical fiber 10 serving as a transmission medium, namely an opticalfiber laid in the transmission section that is positioned between thetransmitter station (or repeater station) and the receiver station (orrepeater station). Particularly, as shown in FIG. 1, one end 10 a of theoptical fiber 10 is optically connected to the transmitter station (orrepeater station) 20, while the other end 10 b is optically connected tothe receiver station (or repeater station) 30. Thereby, the opticalfiber 10 is laid in the transmission section between the transmitterstation (or repeater station) 20 and the receiver station (or repeaterstation) 30. The optical signals are transmitted from the transmitterstation (or repeater stations) 20 to the receiver station (or repeaterstations) 30 via the optical fiber 10. In the optical communicationssystem 1, pumping light for Raman amplification, outputted by a pumpinglight source 31 provided in the receiver station (or repeater station)30, is supplied to the optical fiber 10 via an optical coupler 32. Insuch a configuration, the optical signals aredistributed-Raman-amplified in the optical fiber 10. The opticalcommunications system 1 includes a structure wherein optical signals aretransmitted in accordance with a multilevel modulation scheme of four ormore levels. For example, in the present embodiment, the transmitterstation 20 transmits optical signals to the optical fiber 10 with amultilevel modulation scheme such as QPSK and 16-QAM.

In such an optical communications system 1, an optical fiber havingvarious characteristics is ordinarily used as the optical fiber 10 thatis laid in the transmission section. Besides the optical fibersdisclosed in Documents 1 to 5, a dispersion-shifted optical fiber whichhas, at the wavelength of 1550 nm, a transmission loss of 0.20 dB/km andan effective area of 50 μm², a single mode optical fiber which has, atthe wavelength of 1550 nm, a transmission loss of 0.19 dB/km and aneffective area of 80 μm², or a pure silica central core optical fiberwhich has, at the wavelength of 1550 nm, a transmission loss of 0.16dB/km and an effective area of 110 μm², may be used as the optical fiber10. In the explanation below, the above-described dispersion-shiftedoptical fiber will be used in Comparative example A, the above-describedsingle mode optical fiber in Comparative example B, and theabove-described pure silica central core optical fiber in Comparativeexample C.

FIG. 2 is a graph showing the relationship at the wavelength of 1550 nmbetween transmission loss, effective area and OSNR of optical fibers.The graph shows contour lines of the distribution of the OSNRimprovement amount on a plane defined by setting, at the wavelength of1550 nm, the transmission loss (dB/km) to the ordinate and the effectivearea A_(eff)(μm²) to the abscissa. The OSNR improvement amount denotesthe improvement amount of OSNR, based on the transmission loss reductionand the phase shift reduction of self-phase modulation in a transmissionsection 80 km long, while taking as a reference the OSNR of the opticalfiber according to Comparative example A.

FIG. 3 is a graph showing the relationship at the wavelength of 1550 nmbetween transmission loss, effective area and power of pumping light forRaman amplification of optical fibers. The graph shows, by using contourlines, the power distribution of the pumping light for Ramanamplification on a plane defined by setting, at the wavelength of 1550nm, the transmission loss (dB/km) to the ordinate and the effective areaA_(eff) (μm²) to the abscissa. The power of the pumping light for Ramanamplification denotes the power of the pumping light at a wavelength of1450 nm as required for obtaining a Raman amplification gain that justbalances out the transmission loss at the wavelength of 1550 nm, in atransmission section 80 km long.

In FIGS. 2 and 3, the positions of the optical fibers according toComparative examples A to C are shown, and the range of the opticalfiber 10 that is used in the optical communications system 1 accordingto the present embodiment is also indicated. The transmission loss α(dB/km) and the effective area A_(eff) (μm²) of the optical fiber 10according to the present embodiment satisfy, at the wavelength of 1550nm, the following relationships (2a) to (2c):α≈−0.001·A _(eff)+0.27  (2a)0.13≦α≦0.15  (2b)A_(eff)≧120  (2c)

In the optical communications system 1 wherein such an optical fiber 10is laid in transmission sections, the OSNR can be improved (see FIG. 2)while suppressing the power increase of the pumping light fordistributed Raman amplification (see FIG. 3), as compared with cases inwhich the optical fibers of Comparative example A to C are used.Examples of such an optical fiber 10 are explained next.

FIGS. 4A and 4B show the cross-sectional structure of an optical fiberaccording to a first example that can be used in the opticalcommunications system according to the present embodiment, and therefractive index profile thereof. As shown in FIG. 4A, the optical fiber10A of the first example comprises a central core 11A which has arefractive index n₁ and extends along a predetermined axis (optical axisAX), and a cladding 100A which has a refractive index n₂ (<n₁) andsurrounds the outer periphery of the central core 11A. FIG. 4B is arefractive index profile 150A of the optical fiber 10A, along the lineL1 (line orthogonal to the optical axis AX) in FIG. 4A. In therefractive index profile 150A, a region 151A denotes a refractive indexof the central core 11A along the line L1, and a region 152A denotes therefractive index of cladding 110A along the line L1.

The structural parameters of the optical fiber of the first example areselected so as to achieve optical characteristics that satisfy the aboverelationship equations (2a) to (2c). As described above, the opticalfiber 10A of the first example comprises, for instance, the central core11A, and the cladding 100A which surrounds the outer periphery of thecentral core 11A and which has a refractive index n₂ lower than therefractive index n₁ of the central core 11A, and has, at the wavelengthof 1550 nm, a transmission loss of 0.15 dB/km and an effective area of120 μm². As an example of structural parameter, the relative refractiveindex difference of the central core 11A with respect to the cladding100A is 0.26%, and the diameter of the central core 11A is 10.8 μm. Thefictive temperature of the central core 11A is 1300° C. The fictivetemperature can be realized by controlling the cooling rate duringdrawing and promoting the relaxation of the glass structure of thecentral core 11A.

As shown in FIG. 2, the OSNR improvement amounts of the optical fiberdisclosed in Document 1 (average loss: 0.169 dB/km, effective area: 90μm² based on the disclosure of Document 2) and of the optical fiberdisclosed in Document 3 (loss: 0.184 dB/km, effective area: 120 μm²) arejust under 5 dB. The OSNR improvement amount in the optical fiber of thefirst example, by contrast, is of about 7 dB, i.e. an OSNR improvementamount of 2 dB or more can be realized. The OSNR at the wavelength of1550 nm in a receiver terminal in multi-stage opticalamplification/repeating systems is expressed by equation (3) below. Ascan be seen from the equation (3), the OSNR improvement of 2 dB allowselongating the transmission distance about 1.6-fold. In the equation,P_(in) denotes the optical power per wavelength channel to be inputtedinto the optical fiber, NF denotes the noise figure of the opticalamplifier, L_(sp) denotes the loss of the transmission section, andN_(amp) denotes the number of optical amplifiers (=the number of relaysections−1).OSNR=58+P _(in) −NF−L _(sp)−10·log N _(amp)  (3)

System performance in optical communications systems based on opticalamplification/repeating can be improved by using distributed Ramanamplification. As can be seen from FIG. 3, the power of the pumpinglight for Raman amplification of the optical fiber (loss: 0.184 dB/km,effective area: 120 μm²) disclosed in Document 3 must be about 1.5 timesthat of a standard single mode optical fiber (optical fiber ofComparative example B). In contrast thereto, the power of the pumpinglight for Raman amplification of the optical fiber 10A of the firstexample can be about the same as that of the optical fiber ofComparative example B.

A first configuration of an optical fiber of a second example comprisesa central core having a cross-sectional structure and a refractive indexprofile identical to those of the optical fiber 10A of theabove-described first example (see FIGS. 4A and 4B), and a claddingsurrounding the central core and having a lower refractive index thanthat of the central core. The optical fiber of the second example has,at the wavelength of 1550 nm, a transmission loss of 0.13 dB/km and aneffective area of 140 μm². The central core (first configuration) of theoptical fiber of the second example is preferably comprised of puresilica glass, or silica glass obtained by doping pure silica glass withat least one of P₂O₅ of 1 mol % or more but 10 mol % or less, Cl of lessthan 2000 mol ppm, F of 2000 mol ppm or more but 10000 mol ppm or less,and A₂O (where A is an alkali metal element) of 1 mol ppm or more but10000 mol ppm or less. The pure silica glass contains Cl of 2000 mol ppmor more but 20000 mol ppm or less that is incorporated in thedehydration process during a fiber fabrication. Suitable alkali metalelements in the A₂O are Na, K, Rb and Cs.

A second configuration of the optical fiber of the second example has,for instance, a cross-sectional structure and a refractive index profilesuch as those shown in FIGS. 5A and 5B. Particularly, an optical fiber10B (second configuration) of the second example has a central core 11Bextending along a predetermined axis (optical axis AX) and having arefractive index n₁, and a cladding 100B which surrounds the outerperiphery of the central core 11B, as shown in FIG. 5A. The cladding100B comprises a first cladding 12B which surrounds the outer peripheryof the central core 11B and has a refractive index n₂ (<n₁), a secondcladding 13B which surrounds the outer periphery of the first cladding12B and has a refractive index n₃ (<n₂), and a third cladding 14B whichsurrounds the outer periphery of the second cladding 13B and has arefractive index n₂ (>n₃). FIG. 5B is a refractive index profile 150B ofthe optical fiber 10B, along line the L2 (line orthogonal to the opticalaxis AX) in FIG. 5A. In the refractive index profile 150B, a region 151Bdenotes the refractive index of the central core 11B along the line L2,region 152B denotes the refractive index of the first cladding 12B alongthe line L2, region 153B denotes the refractive index of the secondcladding 13B along line the L2, and region 154B denotes the refractiveindex of the third cladding 14B along the line L2.

As shown in FIGS. 5A and 5B, the refractive index n₁ of the central core11B is highest, and the refractive index n₃ of the second cladding 13Bis lowest, from among the refractive indices of the regions in theoptical fiber 10B. The first to third claddings 12B to 14B are dopedwith F element. The second cladding 13B is comprised of silica glassdoped with F element whose amount is more than the doping amount of Felement in each of the first cladding 12B and the third cladding 14B.Alternatively, the second cladding 13B is suitably comprised of silicaglass having formed therein a plurality of voids extending in the fiberaxial direction (optical axis AX) (see optical fiber 10C of a thirdconfiguration, shown in FIG. 6). The refractive indices of the firstcladding 12B and the third cladding 14B are identical. With respect tothe refractive index (=n₂) of the foregoing, the relative refractiveindex difference of the central core 11B is 0.22%, and the relativerefractive index difference of the second cladding 13B is −0.75%. Thediameter of the central core 11B is 11.9 μm. The diameter of the firstcladding 12B is 20.8 μm. The diameter of the second cladding 13B is 39.0μm. The diameter of the third cladding 14B is 125 μm.

FIG. 6 is a cross-sectional view showing the third configuration of theoptical fiber of the second example. The third configuration differsfrom the above-described second configuration in that now the secondcladding is comprised of silica glass having formed therein a pluralityof voids extending in the fiber axial direction (optical axis AX). Inparticular, the optical fiber 10C of the third configuration comprises acentral core 11C extending along a predetermined axis (optical axis AX)and having a refractive index n₁, and a cladding 100C which surroundsthe outer periphery of the central core 11C. Furthermore, the cladding100C comprises a first cladding 12C which surrounds the outer peripheryof the central core 11C and has a refractive index n₂ (<n₁), a secondcladding 13C which surrounds the outer periphery of the first cladding12C and has a refractive index n₃ (<n₂), and a third cladding 14C whichsurrounds the outer periphery of the second cladding 13C and has arefractive index n₂ (>n₃). In particular, the lowest refractive index(n₃) is realized in the second cladding 13C, from among the regions thatmake up the optical fiber 10C, thanks to the plurality of voids 120formed along the optical axis AX (the optical fiber 10C has the samerefractive index profile as the refractive index profile 150B shown inFIG. 5B).

A transmission loss of 0.13 dB/km can be realized in the optical fiberof the second example (first to third configurations) through loweringof the viscosity of the silica glass and by setting of the fictivetemperature of the central core, in the fiber state, to about 1000degrees. A fictive temperature of 1000 degrees in the central core canbe realized by forming the central core from pure silica glass, orsilica glass obtained by doping pure silica glass with at least one ofP₂O₅ of 1 mol % or more but 10 mol % or less, Cl of less than 2000 molppm, F of 2000 mol ppm or more but 10000 mol ppm or less, and A₂O (whereA is an alkali metal element) of 1 mol ppm or more but 10000 mol ppm orless. Pure silica glass contains Cl of 2000 mol ppm to 20000 mol ppmthat is incorporated in the dehydration process during a fiberfabrication. Suitable alkali metal elements in the A₂O are Na, K, Rb andCs. A fictive temperature of 1000 degrees in the central core can berealized by manufacturing an optical fiber having a central core of puresilica glass, P₂O₅-doped silica glass or the above-describedalkali-metal-doped silica glass, while controlling the cooling rateduring drawing.

Increased macrobending loss and microbending loss become a concern whenthe effective area is expanded to 140 μm². However, macrobending can bekept down at a level similar to that of standard single mode opticalfibers by, for instance, providing the low-refractive index secondcladdings 13B, 13C, such as those shown in FIGS. 5A and 6. Microbendingcan be kept down at a level similar to that of standard single modeoptical fibers by, for instance, improving fiber sheathing.

As can be seen from FIG. 2, the OSNR improvement amount of the opticalfiber of the second example (first to third configurations) is about 9dB, which constitutes an improvement of 4 dB or more over the opticalfibers disclosed in Documents 1 and 3. As shown in FIG. 2 of Mizuochi,2008 IEICE Society Conference, BCI-1-12 (Document 6), an OSNRimprovement of 4 dB enables a transition from QPSK to 16-QAM whilekeeping the error rate constant. Therefore, the optical communicationssystem 1 according to the present embodiment, in which an optical fiberof the second example is laid in a transmission section, allowsrealizing a transmission capacity that is double that of the opticalcommunications systems disclosed in Documents 1 and 3. Moreover, as canbe seen from FIG. 3, the power of the pumping light for Ramanamplification can be kept equivalent to or lower than that of a standardsingle mode optical fiber.

The optical communications system according to the present inventionallows improving OSNR while curbing increases in the power of thepumping light for distributed Raman amplification.

1. An optical communications system in which an optical signal istransmitted via an optical fiber that is laid in a predeterminedtransmission section, and the optical signal isdistributed-Raman-amplified in the optical fiber, wherein a transmissionloss α (dB/km) and an effective area A_(eff) (μm²) of the optical fibersatisfy, at the wavelength of 1550 nm, the following relationships:0.13≦α≦0.15A_(eff)≧120, wherein the optical fiber comprises a central coreextending along a predetermined axis, and a cladding surrounding anouter periphery of the central core and having a refractive index lowerthan that of the central core, and wherein the central core of theoptical fiber comprises pure silica glass, or silica glass obtained bydoping pure silica glass with at least one of P₂O₅, of 1 mol % or morebut 10 mol % or less, Cl of less than 2000 mol ppm, F of 2000 mol ppm ormore but 10000 mol ppm or less, and A₂O (where A is an alkali metalelement) of 1 mol ppm or more but 10000 mol ppm or less.
 2. An opticalcommunications system according to claim 1, wherein the cladding of theoptical fiber comprises a first cladding surrounding the outer peripheryof the central core, a second cladding surrounding an outer periphery ofthe first cladding, and a third cladding surrounding an outer peripheryof the second cladding, and wherein a refractive index of the centralcore is highest, and a refractive index of the second cladding islowest, from among the central core, the first cladding, the secondcladding and the third cladding.
 3. An optical communications systemaccording to claim 2, wherein the second cladding of the optical fibercomprises silica glass doped with F element whose amount is more than adoping amount of F element in each of the first cladding and the thirdcladding, or silica glass having formed therein a plurality of voidsextending in a fiber axial direction.
 4. An optical communicationssystem in which an optical signal is transmitted via an optical fiberthat is laid in a predetermined transmission section, and the opticalsignal is distributed-Raman-amplified in the optical fiber, wherein atransmission loss α (dB/km) and an effective area A_(eff) (μm²) of theoptical fiber satisfy, at the wavelength of 1500 nm, the followingrelationships:0.13≦α≦0.15A_(eff)≧120, wherein the optical fiber comprises a central coreextending along a predetermined axis, a first cladding surrounding anouter periphery of the central core, a second cladding surrounding anouter periphery of the first cladding, and a third cladding surroundingan outer periphery of the second cladding, and wherein a refractiveindex of the central core is highest, and a refractive index of thesecond cladding is lowest, from among the central core, the firstcladding, the second cladding and the third cladding.
 5. An opticalcommunications system according to claim 4, wherein the second claddingof the optical fiber comprises silica glass doped with F element whoseamount is more than a doping amount of F element in each of the firstcladding and the third cladding, or silica glass having formed therein aplurality of voids extending in a fiber axial direction.
 6. An opticalcommunications system in which an optical signal is transmitted via anoptical fiber that is laid in a predetermined transmission section, andthe optical signal is distributed-Raman-amplified in the optical fiber,wherein a transmission loss α (dB/km) and an effective area A_(eff)(μm²) of the optical fiber satisfy, at the wavelength of 1550 nm, thefollowing relationships:0.13≦α≦0.15A_(eff)≧120, and wherein the optical communications system has astructure in which optical signals are outputted by multilevelmodulation of four or more levels.
 7. An optical communications systemin which an optical signal is transmitted via an optical fiber that islaid in a predetermined transmission section, and the optical signal isdistributed-Raman-amplified in the optical fiber, wherein a transmissionloss α (dB/km) and an effective area A_(eff) (μm²) of the optical fibersatisfy, at the wavelength of 1550 nm, the following relationships:0.13≦α≦0.15A_(eff)≧120, and wherein the effective area Aeff (μm²) of the opticalfiber satisfies, at the wavelength of 1550 nm, the followingrelationship:A_(eff)≦140.